A centrifugal fan is a rotating piece of equipment that has vanes or blades contained in a housing or casing. The spinning vanes produce air flows with high volumes, but low pressures. Typically, an electric motor drives the vanes and as the vanes rotate, the air is pushed against the outer walls of the housing and exit at a right angle to the intake. The action of the air being thrown against the walls provides the centrifugal force that drives the air stream.
Determine the maximum volume of air needed for the process. For instance, assume the air volume needed is 5 cubic feet per minute (300 feet per second).
Determine the size of the outlet duct based on air velocity. For example, good engineering practice specifies air velocities to be between 70 feet to 100 feet per second. For this application, assume a velocity of 90 feet per second. The cross sectional area of the duct will be the volumetric flow rate divided by the velocity or 300/90 which is 3.3 square feet. For a square duct, each side can be approximately 2 feet and the outlet of the fan should be 2 feet by 2 feet.
Determine the size of the electric motor to produce the volume required. Fan manufacturers can supply a value called the displacement of air per revolution ("V"). This means for each rotation of the vanes, a specific volume of air is pushed out. A typical rotational speed for an electric motor is 3,500 revolutions per minute (rpm) with an efficiency of 70 percent ("n"). Assume the air displacement is 50 cubic feet per revolution. The equation to determine the motor horsepower (hp) is hp = V x rpm / 396,000 x n. This calculates to be 50 x 3,500 / 396,000 x 0.70 or 0.63 horsepower. Therefore, a 3/4 horsepower motor can be used for this application.
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